1. Field of the Invention
The present invention relates to semiconductor fabrication processes. More particularly, the present invention comprises a method for selective epitaxy using laser annealing. The method has many applications, including production of thin film transistors. Such transistors may be formed over insulators used in Liquid Crystal Display (LCDs).
2. Decription of Related Art
Semiconductor fabrication is a multi-billion dollar industry that has emerged into international importance in the past 10-20 years. An important part of the business is intergrated circuits, including computer chips and many other electrical components which are made from semiconductor material. Integrated circuits are basic building blocks for computers and other electronic equipment.
As the semiconductor industry has grown, improved processing techniques have been sought continually. Improved processing techniques can increase efficiency of the manufacturing process or the improved technique may provide a better product. For competitive businesses, important considerations include the cost of the component, and the yield of the manufacturing processes. The components themselves have been continually refined for enhanced capabilities; today's components are faster and more reliable than the components of the past.
A number of methods have been developed for manufacturing integrated circuits. Generally, fabrication processes begin with a thin wafer of semiconductor crystal material, such as silicon, which is the substrate upon which the intergrated circuits will be manufactured. The actual integrated circuits are formed in a complicated series of steps, typically involving many different processes before the final product is obtained. In the process, many different layers may be built up, including layers of single crystal silicon, polycrystalline silicon, or metal. Masking techniques may be applied to remove selected portions using photoresist or to dope selected portions in order to achieve the desired electrical characteristics in those selected portions of the substrate or layers. Many manufacturing methods are known by those skilled in the art. The result of the manufacturing process is a structure on the micron-sized scale.
An important process is growth of an epitaxial layer, which is a layer of a material grown over a substrate layer. Epitaxial layers may be grown by a number of processes. In "liquid-phase epitaxy" (LPE), the epitaxial layer is grown from liquid that solidifies around the substate. In LPE, the substrate serves as the seed crystal onto which the new crystalline material grows, and also defines the shape of the growth. In "vapor-phase epitaxy" (VPE), a heated substrate is placed in an atmosphere having a vapor containing the constituents of the new epitaxial layer. The vapor crystallizes on a substrate, which acts as the seed crystal. An advantage of these processes is that the pure material can be grown at temperatures well below the melting point of the semiconductor.
In another versatile method for growing epitaxial layers, molecular beam epitaxy (MBE), a molecular beam is directed at the substrate in a vacuum. In MBE, a layer may be made that is a combination of various elements. For example, in the growth of AlGaAs on GaAs substrates, the Al, Ga, and As components, along with the dopants, are heated in separate cylindrical cells. Collimated beams of these constituents escape into the vacuum and are directed onto the surface of the substate. The rates at which these atomicc beams strike the surface can be closely controlled. If MBE is done at a high temperature (about 560.degree. C. for GaAs), then growth of very high quality crystals results. However, if MBE is done at a lower temperature, then the layer is amporphous (non-crystalline) to some extent. In that instance, the resulting amorphous layer is annealed at a high temperature for a specific period of time in order to obtain the desired crystal quality.
Changes in doping or in crystal composition (e.g., the ratio of Al to Ga in Al GaAs) can be obtained by controlling shutters in front of the individual beams. An abrupt change can be accomplished by covering the shutter of a particular constituent. A gradual change can be formed in slowly varying the shutters to obtain the desired result. The flexibility and versatility of the MBE process has made it popular in the semiconductor industry.
Semiconductor doping is a useful process. A dopant is placed in a semiconductor material to vary its electrical characteristics; the dopant resides in the crystalline lattice, but the dopant has slightly different structure from the constituents of the pure semiconductor. A "n-type" semiconductor is produced by using a dopant that has an excess electron, such as phosphorous in a silicon crystal; a "p-type" semiconductor is produced by using a dopant that lacks an electron, such as boron in a silicon crystal. The amount of dopant may be varied to produce lightly doped, or heavily doped semiconductor layers.
Junctions between p-type and n-type semiconductors are fundamental to the performance of amplification, switching, and other important operations in electrical circuits. The idea of a junction is that only the doping is changed; there are no significant changes in the crystal lattice itself. A diode is formed by a single junction of a p-type semiconductor formed adjacent to an n-type semiconductor. The diode generally restricts flow of electrical current to one direction. A bipolar transistor is formed by a combination of n-type, p-type, and n-type layers (a npn transistor) or a combination of p-type, n-type, and p-type layers (a pnp transistor). Another type of transistor, the metal oxide semiconductor field effect transistor (MOSFET), may be formed in a p-type substrate by forming two n-type areas, separated by a gate. In operation, a voltage placed across the gate allows current flow between the two n-type areas.
A junction of semiconductor material may be formed by growing of an epitaxial layer of a semiconductor that is appropriately doped. Epitaxial layers have been described above. Other methods of junction fabrication include difusion and ion implantation.
In the diffusion method, a gas having a high concentration of the dopant atoms is diffused into a semiconductor crystal. For example, a wafer of n-type semiconductor crystal may be placed in a furnace at 1000.degree. C., in a gaseous atmosphere containing a high concentration of boron atoms (a p-type dopant). The boron atoms will diffuse into the surface of the n-type silicon and spread a limited distance into the adjoining silicon by random motion. Thus, looking at the impurity profile, the highest concentration of boron atoms will appear near the surface. The p-type doping concentration will decrease further from the surface, until at the junction the boron concentration decreases below the level of n-type doping. A disadvantage of the diffusion process is that the impurity profile is gradual, not abrupt. The gradualness of the impurity profile causes parasitic electrical resistance in the diffused layer. Another disadvantage of the diffusion process is the high temperature that must be maintained in the diffusion oven. Some materials that are useful in intergrated cirucit fabrication, such as aluminum, melt at temperatures well below 1000.degree. C., and therefore cannot be used together with a diffussion process. Furthermore, the high temperature can cause unwanted diffusion in other portions of the circuit.
In the ion implantation method of doping, energetic ions of the dopant are implanted directly into the semiconductor crystal. A beam of the dopant ions is accelerated to kinetic energies ranging from several keV to several MeV, and is directed to the semiconductor crystal. The dopant ions enter the crystal, and come to rest at an average depth. An advantage of the ion implantation process is that it can be done at relatively low temperatures. Furthermore, selective doping is straightforward; the ions are blocked by metal or photoresist layers. Doping can occur to depths as shallow as tenths of a micron. The impurity profile is more defined using the ion implantation process than by using the diffusion method. However, the diffusion profile is still not perfect; the junction created is not as abrupt as would be preferable for many applications. As another disadvantage, the ion implantation process damages the crystal lattice. This damage can be cured by annealing, which is a heating of the crystal to reform the crystal structure. However, some materials useful in integrated circuit fabrication either cannot be subjected to annealing temperatures, or require special treatment to withstand such temperatures. Also, the annealing process can cause unwanted dopant diffusion, which reduces the abruptness of the junction. As an alternative to raising the temperature of the entire circuit, the surface can be heated locally with a laser.
In a less common doping process, Gas Immersion by Laser Doping (GILD) a laser heats only the surface of a semiconductor substrate that is disposed in an atomosphere having the dopant gas. For example, a laser such as an ultraviolet excimer laser is made incident upon a semiconductor surface. The incident laser radiation heats the material, causing it to melt. The laser pulse can be adjusted in duration and intensity, and thus, the depth of the melt can be precisely controlled. The gas diffuses uniformly in the melt. The separation between the melt and the contiguous solid surface is well defined, which allows the melt to be doped without substantially affecting the surrounding solid material. Thus, when the melt is cooled the junction formed between the differently doped surfaces is very abrupt, a desirable characteristic. As an additional benefit, the cooling melt forms a high quality crystalline lattice, thereby eliminating the high temperature annealing process required with conventional doping techniques such as ion implantation.
In all of the many manufacturing steps, the epitaxial processes, the subsequent masking and removal of unmasked portions, the processes must be carefully controlled and monitored; the complexity of the circuit and the smallness of the integrated circuits require exacting precision in the fabrication processes. Manufacturing is typically performed in "clean" rooms that are specially designed with air filtration systems to remove substantially all the dust in the room and thereby prevent contamination of the integrated circuit during manufacture.
Heterostructure devices comprise a number of layers of semiconductor materials, the layers being stacked. The heterostructure configuration is often used for laser diodes. Heterostructure devices are fabricated in a series of steps beginning with a wafer, which is used as a substrate. Each succeeding layer is grown on the top of the other to form the desired combination of layers. Common methods of growing the layers include MBE, ultra-high vacuum chemical vapor deposition (CVD), and limited reaction processing (LRP). The individual heterostructure device are then separated from the wafer by conventional scribing and cleaving techniques.
A pertinent electriccal characteristic of semiconductor materials is their bandgap, which is defined buy the spacing of the energy bands in an energy diagram. This characteristic is not varied by changing the doping concentration as described above. However, the material's bandgap can be varied by combining different semiconductor materials, as in the heterostructure described above. For example, a mixed layer of germanium (Ge) and silicon (Si) may be grown on a silicon substrate. Such a mixed layer can be described by the term Ge.sub.x Si.sub.1-x, with "x" referring to the germanium fraction of the layer. Such layers have been grown in bulk, over an entire surface as in the heterostructure described above. However, no method has been available to grow mixed layers in selective regions. If such a method were available, it would provide a semiconductor fabrication that has many applications, for example in fabrication of diodes, or fast, thin base bipolar transistors.
Bipolar transistors are basic building blocks of many common computer chips. The bipolar transistor comprises an emitter, a base, and a collector. Iin operation, the transistor's output is controlled by electrical current through the base. The characteristics of the base, such as its bandgap, its width, and the amount of doping substantially affect the transistor's operating characteristics such as its frequency response, and the maximum speed at which the transistor can be switched. Bipolar transistors may be of the npn type, or the pnp type. Speed of the transistors is enhanced if the base is narrow and highly doped, thereby providing less resistance. However, gain of the transistor is increased by lighter doping of the base. Therefore, it would be an advantage to provide a process for manufacturing a transistor base that is narrow, heavily doped, and has a high gain.